The present invention relates to heat transfer element baskets and, more specifically, to an assembly of heat absorbent plates in a basket for use in a heat exchanger wherein heat is transferred by means of the plates from a hot heat exchange fluid to a cold heat exchange fluid. The present invention has particular application in heat transfer apparatus of the rotary regenerative type wherein the heat transfer element is heated by contact with a hot gaseous heat exchange fluid and thereafter brought in contact with a cool gaseous heat exchange fluid to which the heat transfer element gives up its heat.
One type of heat exchange apparatus commonly used for gas-to-gas heat exchange in the process industry and for gas-to-air heat exchange on utility steam generators is the well-known rotary regenerative heat exchanger. Typically, a rotary regenerative heat exchanger has a cylindrical rotor divided into sector-shaped compartments in which are disposed a mass of heat transfer element which, as the rotor turns, is alternately exposed to a stream of heating gas and then upon rotation of the rotor to a stream of cooler air or other gaseous fluid to be heated. The heat absorbent mass typically comprises a plurality of heat transfer element basket assemblies mounted in sector shaped compartments. Each of the heat transfer element basket assemblies houses a plurality of heat transfer plates which when exposed to the heating gas absorb heat therefrom and then when exposed to the cool air or other gaseous fluid to be heated, the heat absorbed from the heating gas by the heat transfer plates is transferred to the cooler gas.
Most commonly, such an element basket comprises a frame-like housing having a plurality of sheet-like heat transfer plates disposed therein. Typically, the element basket housing comprises a frame formed of a pair of spaced plate-like end members held together by paired side straps interconnecting the end members along the sides thereof such as shown in U.S. Pat. Nos. 3,314,472, 4,561,492 and 4,606,400. A plurality of heat transfer plates are stacked in closely spaced relationship within the basket housing to provide a plurality of passageways between adjacent plates through which the heat exchange fluids pass. The side straps which interconnect the spaced end members typically extend in pairs along the opposite sides of the stacked array of heat exchange elements. On each side of the heat exchange element is a first side strap extending between the upper regions of the spaced end members and a second side strap extending between the lower region of the end members in spaced, parallel relationship to the first side strap. The side straps may be flanged inwardly along the longitudinal edge lying at the edge of the basket assembly to provide a retaining surface for preventing the heat transfer plates from falling out of the open ends of the element basket as shown in U.S. Pat. No. 3,314,472.
Typically, a plurality of retaining bars are welded between the end plates across the top and bottom ends thereof to further assist in keeping the heat transfer element plates from falling out of the open ends of the element basket. The retaining bars may merely be disposed to lie across the top and bottom edges of the heat transfer element plates as shown in U.S. Pat. No. 4,561,492. Alternatively, in order to provide a shorter basket for a given plate height, the retaining bars may be disposed within recesses cut in the top and bottom edges of the heat transfer element plates as shown in U.S. Pat. No. 4,606,400.
The retaining bars also serve as structural members for supporting lifting means to facilitate handling of the assembled element baskets and, in particular, to facilitate the installation and removal of the element baskets from the heat exchanger. Typically, the lifting means comprises a pair of spaced aparted holes formed in a centrally located retaining bar as shown in U.S. Pat. No. 4,552,204, or a pair of spaced aparted pins integral with and passing through a centrally located retaining bar as shown in U.S. Patent 4,557,318. When the lifting means comprises a pair of holes in the aforementioned U.S. Pat. No. 4,552,204, the element basket is lifted by means of a pair of clevis means which are disposed to span the central retaining bar about each lifting hole. Each clevis is engaged to the retaining bar by a pin which is passed through the lifting hole and each side of the clevis spanning the bar. When the lifting means comprises a pair of lifting pins as in the aforementioned U.S. Pat. No. 4,557,318, the element basket is lifted by means of a pair of lifting lugs which simply grasp the pins extending through the central retaining bar.
Generally, such element baskets are in the configuration of a trapezoidal frustrum. However, the element basket disposed in the radialy outward sections of the sector shaped compartments of the cylindrical rotor necessarily can not be of a trapezoidal cross-section due to the curvature of the surrounding rotor. If these peripheral baskets were of a true trapezoidal cross section there would exist a gap between the radially outward end of the basket and the cylindrical wall of the surrounding rotor. Such a gap is undesirable as gas and air flowing through the rotor and passing through the gap would bypass the heat transfer surface housed in the baskets thereby reducing the efficiency of the heat exchanger. Additionally, the existence of the gap means the rotor is not fully filled with heat transfer surface.
Accordingly, it is customary in the prior art to construct the peripheral baskets in the form of a frustrum having a cross-section shaped as a trapezium, i.e., as a quadrilateral having no parallel sides, rather than as a trapezoid wherein the end members of the basket are parallel to each other. Further, it is customary to provide an arcuate end member as the radially outward end of such a peripheral basket with the arcuate member having a curvature commensurate with the curvature of the surrounding rotor.
In such peripheral baskets, the element sheets disposed within the basket at the radially outward extend thereof must be foreshortened due the curvature of the outer end member. As these foreshortened elements sheet do not extend across the entire width of the basket, they are not adequately held in place by a tight fitting between the side straps as in the case of the fully extending sheets in the remainder of the basket. Therefore, in order to prevent these foreshortened sheets from becoming too loose and shifting around or flexing under the force of high velocity jets of cleaning media during soot blowing or water washing, it is common practice in the prior art to install retaining clips on each of these foreshortened element sheets in peripheral basket assemblies. Each clip must be hand installed and welded to the arcuate end member in such a position as to engage one of the foreshortened element sheets. As two clips must be provided for each foreshortened element sheet, one at the top side of the basket and one at the bottom side of the basket, the process of assembling such peripheral basket assemblies is labor intensive and time consuming.